Microwave assisted magnetic recording, MAMR, is one of several future technologies that are expected to extend perpendicular magnetic recording beyond 1 terabit per square inch. In this scheme, a field generator is placed in close proximity to the write element to produce a high frequency oscillating field in the media film plane. The frequency range of the oscillating field could be from 5 GHz to 50 GHz. Because of ferromagnetic resonance (FMR), it becomes possible to switch media grains at fields below their normal coercivity i.e. a lower write field may be used, but only in the immediate vicinity of the microwave assisted write element.
A microwave field generator is basically a spin torque oscillator (STO). It resembles a current-perpendicular-to-plane (CPP) GMR or TMR structure in that the current flows perpendicular to the film, passing through a spacer layer into a reference layer. The magnetization directions in the stack are, however, different from those of a CPP GMR/TMR sensor.
As schematically illustrated in the top section of FIG. 1, the simplest configuration for a STO is a tri-layer stack consisting of a spin injection layer (SIL) 12, non-magnetic spacer 16 (metallic or insulating), and field generating layer (FGL) 13. The SIL magnetization must be kept perpendicular to the plane of the film, either by an external magnetic field or through its intrinsic perpendicular magnetic anisotropy (PMA). As electrons in oscillating stack 14 transit SIL 12 their spins become polarized by the magnetization present in SIL 12. The degree of spin polarization is further enhanced by the greater probability of electrons, whose magnetization is in the same direction as reference layer 17, being able to transit spacer 16. Spin torque oscillation then occurs in the FGL 13, resulting in the generation of microwaves.
The lower section of FIG. 1 shows how the STO is positioned as part of a perpendicular magnetic write head for use in MAMR. The STO (rotated 90° relative to its orientation in the upper section of FIG. 1) is positioned with its top electrode 11 up against main pole 22 and it bottom electrode 18 up against trailing shield 21. When data is to be written, microwaves emerging from the STO illuminate recording medium layer 20 (seen above soft magnetic underlayer 19) just ahead of main pole 22.
A paper by C. Slonczewski [1] on spin-transfer torque (STT) magnetization switching has attracted considerable interest due to its potential application to spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. [2] described another spintronic device called a spin transfer oscillator where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. See FIG. 1 above.
In two recent patent applications by Headway [3] [4], it was shown that a large perpendicular magnetic anisotropy (PMA) can be established in both the [Co/Ni]xn and [CoFe/Ni]xn multilayer systems by using a seed layer of Ta/Ru/Cu in combination with a relatively low total thickness. Thus an effective MAMR device can be fabricated with these [Co/Ni] and/or [CoFe/Ni]xn multilayer PMA systems.
Furthermore, it was shown that the performance of FGLs such as FeCo, FeCoAl, etc can be improved by inserting a magnetic layer such as a [Co/Ni]xn multilayer or a [CoFe/Ni]xn multilayer having PMA. However, these patent applications do not cover all possible schemes for the top STO case. The present invention discloses particular STO structures that utilize [Co(Fe)/Ni] multilayer PMA coupling with a high saturation magnetization (Ms) material such as FeCo to form the FGL.    [1] C. Slonczewski, “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996)    [2] J. Zhu et al, “Microwave Assisted Magnetic Recording”, IEEE. Trans. Magn. 44, 125 (2008)    [3] K. Zhang, et. al. Headway application Ser. No. 12/456,621    [4] K. Zhang, et. al. Headway application Ser. No. 12/800,196
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 7,616,412 (Zhu et al) discloses a reference stack comprising both perpendicular and in-plane components for microwave assisted magnetic recording while U.S. Pat. No. 7,352,658 (Shimazaki et al) shows a first magnetic film having perpendicular anisotropy and a second magnetic film having either perpendicular or in-plane anisotropy.
In regard to U.S. Pat. No. 7,616,412 (Zhu et al.), this invention proposes to cover a reference layer system with a weaker perpendicular magnetic anisotropy (PMA). An example would be a system with both in-plane and out-of-plane anisotropies, but with the out-of-plane anisotropy being greater than the in-plane anisotropy, thereby tilting the magnetization partially out of the plane. As will become apparent below, this teaches away from the present invention where the in-plane and out-of-plane anisotropies are in different layers. Zhu et al. speculate that, even when the anisotropy is partially tilted in-plane, they can still have adequate signal readout without a sensor because of the in-plane component of the anisotropy. This suggests that without tilting, the readout would be zero because both the oscillating and the reference layers are parallel to each other (CPP GMR=0).